This invention relates to propylene resin compositions, and more particularly to propylene resin compositions having a well-balanced combination of transparency, stress-whitening resistance and impact resistance at low temperatures, and further having good heat resistance.
Polypropylene resins have been used in a wide variety of fields, because or their relatively low cost and excellent thermal and mechanical characteristics. In general, however, propylene homopolymers have high stiffness, but poor impact resistance, especially at low temperatures. To improve the impact resistance at low temperatures of propylene homopolymers, a propylene block copolymer composition comprising a propylene homopolymer component as initially produced and an ethylene-propylene random copolymer component as subsequently produced has been extensively employed in various industrial fields including automobiles and household appliances.
These conventional propylene block copolymer compositions are excellent in impact resistance, but inferior in transparency as compared with homopolymers, and have large whitening when subjected to impact. As a method to improve the disadvantage of whitening when subjected to impact in the propylene block copolymer composition, there have been proposed a method of increasing the ethylene content in the copolymer and a method of adding polyethylene to the propylene block copolymer composition. Both methods are excellent in improving impact resistance, but have a problem, i.e, they reduce the transparency of the product.
JP-A-5-331327 discloses a polymer composition comprising a propylene block copolymer composition with a specified ratio of the intrinsic viscosity of a propylene homopolymer component to that of an ethylene propylene random copolymer component. JP-A-6-145268 discloses a polymer composition with a specified intrinsic viscosity of a propylene homopolymer component, a specified ratio of the intrinsic viscosity of a propylene homopolymer component to that of an ethylene propylene copolymer component and a specified ethylene content in the ethylene propylene random copolymer component. JP-A-56-72042 and JP-A-57-63350 disclose a polyolefin resin composition wherein an ethylene-propylene copolymer containing a small amount of ethylene is blended with another ethylene-propylene copolymer. JP-A-10-87744 discloses a propylene resin composition wherein a small amount of ethylene is incorporated in a propylene homopolymer component in the ethylene-propylene block copolymer.
Shaped articles made from these polymer compositions have been more improved than conventional propylene block copolymer compositions in respect of the stress-whitening resistance and transparency, but additional improvements have been required.
An object of the invention is to provide a propylene resin composition having a well-balanced combination of transparency, stress-whitening resistance and impact resistance at low temperatures and further heat resistance characteristics.
As a result of earnest investigations to attain the above-described objects, we have found that a propylene resin composition wherein (A) a propylene-xcex1-olefin random copolymer containing a specified amount of xcex1-olefin unit and (B) a propylene-xcex1-olefin random copolymer having the content of xcex1-olefin unit different from the random copolymer (A) are composed in a specified composition has a well-balanced combination of transparency, stress-whitening resistance and impact resistance at low temperatures, and further good heat resistance characteristics. The temperature dependence of dynamic viscoelasticity of the resin composition is such that, the loss tangent (tan xcex4) and the storage elastic modulus (Exe2x80x2) meet certain conditions. The present invention was thus completed.
Thus the present invention relates to a propylene resin composition comprising 80 to 40% by weight, based on the weight of the composition, of (A) a propylene-xcex1-olefin random copolymer with the content of a propylene unit of 99.1 to 99.9% by weight, and 20 to 60% by weight, based on the weight of the composition, of (B) a propylene-xcex1-olefin random copolymer with the content of a propylene unit of 70 to 90% by weight, and the temperature dependence of dynamic viscoelasticity of the composition is such that the composition shows only one peak of loss tangent (tan xcex4) in the temperature range of xe2x88x9280xc2x0 C. to 80xc2x0 C., and the temperature at which the storage elastic modulus (Exe2x80x2) is 1xc3x97108 dyn/cm2 or less is not less than 150xc2x0 C.